How do you know when to transition your kid to 1 nap?

I transitioned my kids to one nap after 2 years of age. The triggering point for me was that with 2 naps they had way too much energy close to bed time in the night. They refused to settle down for a nap until 10pm! This drove me a bit insane as I had a full and tiring day of work and doing household chores that I needed a break too. So I reduced their nap to just one and it did the trick. By 7.30pm they were tired and ready for bed. Win-win!